Method of making polarized ophthalmic glass

ABSTRACT

Polarized ophthalmic glass lenses are made from conventional ophthalmic glass. This is accomplished by heating a sheet of ophthalmic glass, which includes a reducible metal oxide as part of its composition, to its softening point in a reducing atmosphere for a time interval sufficient to reduce the metal oxide to metal to a predetermined depth on at least one surface of the sheet. Following this reduction of the metal oxide, the sheet is held at an elevated temperature to permit the reduced oxides to nucleate. Then, the sheet is stretched in one direction to elongate the nucleated metal particles in parallel lines. The glass then is shaped, cut into lenses, permitted to cool, and the outer surface of the lens blanks are ground and polished in a conventional manner, leaving the stretched elongated metal particles on the inner surface thereof to create polarized ophthalmic lenses.

BACKGROUND OF THE INVENTION

In order to more fully understand the invention disclosed and claimedherein, a brief discussion of polarization of light is consideredhelpful. Light generally travels in a transverse direction with electricvibrations perpendicular to the line of propogation of the light waves.Light is polarized linearly and horizontally when the electricalvibrations are horizontal; and when the vibrations are vertical, thelight is considered to be polarized linearly and vertically. Thus, if abeam of light is passed through a first polarizer which divides thelight into two components, one transmitted or passed through thepolarizer while the other one is blocked, the remaining light has eitherhorizontal or vertical polarization. If this polarized lightsubsequently is passed through a second polarizer maintained parallel tothe first one, the polarized light all is transmitted through the secondpolarizer. If, however, the second polarizer is rotated, the amount oflight passed through it decreases proportional to the amount of rotationof the second polarizer. When the polarizers are at right angles to oneanother, all of the light theoretically is absorbed by the secondpolarizer.

This phenomenon is employed to substantial advantage in the use ofpolarized sunglasses to substantially reduce the annoying effects ofglare, since reflected sunlight (glare) has its polarization rotatedninety degrees or at right angles to direct sunlight. The most commonpolarized sunglasses are made of stretched plastic material which haslong, thin, parallel chains of iodine or similar material embedded in itto permanently polarize it. Sunglasses made of this material have becomevery popular, but suffer from a number of inherent disadvantages. Theplastics have a low hardness, and therefore, a poor scratch resistence;so that unless a great deal of care is taken to avoid scratching them,lenses made of such plastics rapidly deteriorate to the point where theyare unusable or should not be used by the wearer. In addition, theseplastics have a low refractive index which prevents manufacture ofprescription polarized sunglasses from them.

Prescription sunglasses have been developed using photochromic glasses,which include submicroscopic crystals of silver halides, such as silverchloride, silver chromide, or silver iodide, which become darker incolor when the glass is subjected to actinic radiation, but which regaintheir original color (or clarity) when the radiation is removed orreduced. Such a photochromic glass is disclosed in the patent toArmistead et al, U.S. Pat. No. 3,208,860. A later patent to Hares et al,U.S. Pat. No. 4,190,451, discloses a photochromic glass which isdescribed as also having the capability of being either thermallytempered or chemically strengthened to comply with present regulationsin existence for use in ophthalmic applications. The glasses disclosedin both of these patents, however, are not polarized glasses, but simplyexhibit the characteristics of becoming darker when exposed to actinicradiation, and then fading or returning to their original color whensuch radiation is removed.

The formation of photochromic glass of the type disclosed in theArmistead and Hares et al patents requires the deliberate introductionof silver halides into the glass along with small amounts of reducingagents. Bulk processing of the glass takes place since the silver halidecrystals and the reducing agents are uniformly dispersed throughout theglass.

An improvement in the photochromic glasses described above is disclosedin the patents to Simms, U.S. Pat. No. 3,892,582, and U.S. Pat. No.3,920,463. These patents both disclose processes for permanently tintingphotochromic glass by heating the photochromic material in a reducingatmosphere, and while it is at an elevated temperature, irradiating itwith ultraviolet irradiation. The change in the tint of the photochromicmaterial is caused by the heating and is emphasized or darkened by thesubsequent ultraviolet irradiation. The formation of the glassesdescribed in the Simms patents includes the introduction of silverhalides into the glass batch in a manner similar to the production ofthe photochromic glasses described in the Armistead and Hares et alpatents. The improvement is in the introduction of a permanentoverriding tint of varying intensity coupled with the photochromiccharacteristics of the glasses.

An effort to combine photochromic characteristics with polarization inophthalmic lenses, and the like, is disclosed in the patent to Araujo,et al, U.S. Pat. No. 3,653,863. The glass described in the Araujo patentis made by introducing crystalline silver halide into the glass batchalong with a small amount of low temperature reducing agents in thebatch. When the batch is subjected to heat, the reducing agents arecatalyzed and operate to reduce the silver halides to metal.Submicroscopic droplets are formed; and as the glass remains at elevatedtemperatures after the reduction, these droplets agglomorate to formlarger balls or masses of silver (silver halide) droplets. The glassthen is stretched to elongate the particles causing elongated fibrilsall aligned in the same direction to be formed throughout the glassbulk.

Because a bulk process is employed in the Araujo patent, it is somewhatdifficult to control the transmission characteristics of the completedlens. For example, it is possible for several fibrils of the elongateddroplets to be aligned with or nearly aligned with one anotherthroughout the thickness of the lens. The only fibrils, however,required for the polarization effect are the outer-most ones where thelight enters the lens. The remaining fibrils simply reduce thetransmission of light through the lens due to scattering and the like.This difficulty is inherent in the bulk effect technique which isemployed in the Araujo method. The aligned lines of polarizing materialare noncontributors to the polarizing effect and simply contribute totransmission losses. All of this is particularly significant if theintent is to manufacture a "clear", high transmission prescriptionophthalmic lens or a photochromic lens where ranges of transmission haveto be carefully controlled as the photochromic lens in a sunglass.

Accordingly, it is desirable to manufacture polarized ophthalmic lensesof high quality without the above disadvantages.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an improved polarizedglass.

It is another object of this invention to provide a method for making animproved polarized ophthalmic glass.

It is yet another object of this invention to make polarized ophthalmicglass using the same composition ordinarily employed for non-polarizedophthalmic glasses.

It is a further object of this invention to provide surface polarizationfor ophthalmic glasses.

In accordance with a preferred embodiment of this invention, a method ofmaking polarized ophthalmic glass includes the step of first heating asheet of ophthalmic glass, which includes a reducible metal oxide aspart of its composition, to its softening point in a reducing atmospherefor a period of time sufficient to reduce the metal oxide to metal to apredetermined depth on at least one surface of the sheet. The glasssheet then is stretched in one direction to elongate the metal particlesin parallel lines. After the stretching has been completed, the glass iscooled to set the elongated metal particles in the glass.

In a more specific embodiment for making ophthalmic lenses, the glasssheet is placed over a shaping fixture while the glass sheet is still atthe softening temperature to permit the sheet to sag and conform to thecurvature of the shaping fixture. The shaping fixture itself has anumber of curved surfaces on it to form the curvature necessary for acorresponding number of lens blanks. After the sheet has conformed tothe shaping fixture, the individual lenses are cut from the fixture. Theglass then is permitted to cool to set the elongated metal particles inthe glass of each of the individual lenses thus formed.

DETAILED DESCRIPTION

In accordance with the method of the present invention, any glass batchcontaining a reducible oxide and suitable for making ophthalmic lensesmay be polarized without changing the starting composition of theophthalmic glass batch in any manner whatsoever from present commonlyused commercial glass compositions. A typical glass, which is wellknown, has the following composition:

    ______________________________________                                                        PERCENT BY WEIGHT                                             COMPONENT       (APPROXIMATELY)                                               ______________________________________                                        SiO.sub.2       32                                                            Na.sub.2 O      1                                                             K.sub.2 O       6                                                             Al.sub.2 O.sub.3                                                                              4                                                             ZnO             1                                                             TiO.sub.2       2                                                             BaO             1                                                             PbO             51                                                            ZrO.sub.2       1                                                             AS.sub.2 O.sub.3                                                                              0.5                                                           Sb.sub.2 O.sub.3                                                                              0.5                                                           ______________________________________                                    

Without degrading the ophthalmic characteristics of the glass in any wayand without altering the desirable light transmission characteristics ofsuch glass, it has been found that such standard ophthalmic glass (andother similar standard ophthalmic glass compositions) can be permanentlypolarized in a controlled and effective manner by heating the glass in areducing atmosphere, permitting the reduced metal oxides (particularlylead oxide reduced to lead metal) which are formed to nucleate, and thenstretching the glass ten to thirty times the original length while it isin the softened state to elongate the reduced metal particles. Afterthis, the glass is allowed to cool and the stretched metal particlescause permanent surface polarization to take place.

The temperature to which the glass must be heated varies dependent uponthe characteristics of the glass batch itself. Typically, such atemperature is between 300° C. to 600° C., or perhaps even above. Thenucleation occurs at all of these temperatures, but the nucleation isfaster at the higher temperatures. Ideally, the elongation or stretchingof the glass to form the polarization lines of stretched lead typicallyis done at the minimum softening temperature for the particular glassformulation which is used.

The exact identity of the reducing atmosphere is not particularlycritical (so long as it is gaseous at the processing temperature, ofcourse), and reducing atmospheres of the type commonly used in the artare used with success in accomplishing the reduction of the metal oxidesin the glass. Similarly, the temperature is not particularly critical,except that at higher temperatures the reduction and nucleation occursmore rapidly than at lower temperatures. As stated above, it also isdesirable to effect the stretching of the glass at or near its lowestsoftening temperature in order to most effectively utilize the shearcharacteristics of the glass in stretching the metal particles.

In selecting the particular reducing atmosphere which is used, cost andsafety are primary factors. Preferred reducing gases include hydrogen,carbon monoxide, cracked ammonia, and similar gases which may be used inpure form or mixed with an inert carrier gas. Because of its readyavailability, hydrogen generally is employed as the reducing atmosphere.Although it is apparent that pure hydrogen may be used, the high dangerof explosion and the relatively high cost of pure hydrogen as comparedto many inert carrier gases, generally dictates the use of hydrogen incombination with an inert carrier gas. For practical purposes, the inertgas used is generally nitrogen because, again, it is readily availableat relatively low cost. Obviously, oxygen should be kept out of thesystem to avoid the danger of explosion even when an inert gas carrieris used in conjunction with the hydrogen gas.

The ratio of the reducing gas to the inert gas carrier is not criticalso far as the manner in which the process functions is concerned. From apractical standpoint, however, if extremely low proportions of reducinggas are employed, the process time is significantly increased withoutany accompanying benefit and results. Because of the time increase forlow proportions of reducing gas, the cost of processing a given batch ofglass is also increased and this is undesirable. It has been found thata ten percent (10%) hydrogen/ninety percent (90%) nitrogen (by volume)reducing atmosphere offers good results at reasonable processing timeswith a minimum of safety hazards. Actually, a range of five percent (5%)hydrogen/ninety percent (90%) nitrogen to fifteen percent (15%)hydrogen/eighty-five percent (85%) nitrogen is probably an ideal workingrange for the reducing atmosphere.

To minimize the danger of hydrogen build-up, if hydrogen is used as theactive reducing component, and further to avoid temperature variationsover the surface of the glass being processed, it is preferable to flowthe reducing atmosphere over the surface of the glass under a slightpositive pressure in either a semicontinuous or continuous system.Consequently, the excess reducing atmosphere is used to constantly flushthe processing apparatus, avoiding hot spots on the surface of the glassand at the glass/reducing atmosphere interface. Highly turbulentconditions should be avoided, since these might cause "hot spots" or"cold spots" on the glass surface. To accomplish this, the pressure ofthe reducing atmosphere generally is maintained only slightly in excessof atmospheric pressure to ensure an even flow over the glass articlesbeing processed.

The method of making ophthalmic polarized glass in accordance with theteachings of this invention can be practiced in batch, semicontinuous,or a continuous manner. Initially, the invention has been practiced inbatch operations; but in full scale commercial operations, continuousprocessing is preferred.

By processing the glass as discussed above, it should be noted that thereduction process is confined to the immediate surface of the glasssheet or blank. Penetration typically is on the order of three to fivemicrons; so that for the completed article, the stretched alignedpolarizing medium is also confined to the surface. Typically, ophthalmiclenses are formed from ophthalmic blanks having the general overall lensshape. To complete a lens, the blank then needs contouring, either aprescription contour or a plano-plano contour, grinding and polishing;and, finally, the overall lens is shaped to a particular frame geometryto create the finished glasses.

Because of the substantial working of the surface of ophthalmic lensblanks, the polarizing process cannot be applied to either a prefinishedblank or the finished lens. In the case of the blank, the grinding andpolishing operations required for finishing would eliminate the surfacepolarizing medium from the lens. For finished lenses, the stretchingstep which necessarily must be made in order to create the polarizinglines in the glass would grossly distort the finely processed curvatureof the lens.

Consequently, it has been found that the reduction and stretching of theglass must be introduced into the ophthalmic glass at a unique point inits processing. Instead of working on the lens blanks, the process isapplied to planar sheets of ophthalmic glass. The thickness of theseglass sheets is selected so that after it is stretched, the finalnecessary thickness for lens blanks is maintained. In addition, itshould be noted that since there is always a grinding and finishing stepnecessary for the creation of prescription lenses, one surface of theplanar sheet must be finished in a manner which precludes the necessityof any further polishing on that surface. Consequently, one surface(selected to be the inner surface of the finished lenses) is alreadyprovided with a final polished finish prior to the heating of the sheetin a reducing atmosphere and its subsequent stretching. Then at theproper temperature, the polarized sheet is placed with this surface overforming molds which shape the inner contour for the lens blanks. Thisinner contour, of course, is polarized and is the finished side of thesheet glass. The other or outer surface also is polarized; but since itis processed further by grinding and polishing prescription lenses, thepolarization is removed from that surface for the creation ofprescription lenses. The back or inner finished surface, however,remains polarized to accomplish the desired purposes.

In the case of the forming of plano-plano ophthalmic lenses or evenplain sunglasses, the final finish can be applied to both surfaces ofthe sheet glass prior to its heating in a reducing atmosphere,stretching, and shaping over the mold; so that both surfaces will bepolarizing surfaces in the completed lenses.

The process also may be applied to photochromic ophthalmic lenses of thetypes disclosed in the prior art.

A variation on the above manufacturing technique which may be eployed isto polarize a very thin sheet of plate glass separately from theprescription lens blanks. This very thin polarized glass sheet then isformed to the inner layer of the prescription semifinished lens, or itmay be formed to the final outer surface, or both surfaces. The finalprocessing step in such a method would be to fuse the thin polarizedglass sheet to the ophthalmic lens by a suitable technique.

The invention is further illustrated by the following examples:

EXAMPLE I

A rectangularly shaped sample of glass three inches (3") by three inches(3") by one-fourth inch (1/4") was obtained having the following glasscomposition:

    ______________________________________                                        INGREDIENT        WEIGHT PERCENT                                              ______________________________________                                        SiO.sub.2         55.9                                                        Al.sub.2 O.sub.3  9.0                                                         B.sub.2 O.sub.3   16.2                                                        LiO               2.65                                                        NaO               1.85                                                        PbO               5.05                                                        BaO               6.7                                                         ZnO               2.3                                                         Ag                0.16                                                        Cl                0.29                                                        Br                0.72                                                        CuO               0.036                                                       F                 0.2                                                         ______________________________________                                    

The sample was polished to a finished surface on its lower side. Thesample was heated to a temperature of 500° C. and then subjected to areducing atmosphere consisting of ten percent (10%) hydrogen and ninetypercent (90%) nitrogen for a period of ten (10) minutes.

The ten percent (10%) hydrogen/ninety percent (90%) nitrogen atmospherewas then removed and replaced by a non-reducing atmosphere purge of onehundred percent (100%) nitrogen, and the sample was held in thisatmosphere at the same temperature (500° C.) for another period of two(2) hours to nucleate the reduced oxides.

The glass sample was clamped to a fixed clamp at one end and heated toits softening temperature range (approximately 600° C.) at which rangeit began to deform (stretch) under its own weight. This stretching wasallowed to continue until the sample was stretched to an overall lengthof forty (40) inches.

The stretched glass then was placed over a lens shaping fixture, heatedto near the softening temperature of the glass, and was permitted to sagto conform to the desired lens curvation provided by the shapingfixture. The lens blanks then were cut from the fixture and permitted tocool.

Polarization efficiency was measured and found to be forty percent (40%)effective.

EXAMPLE II

The rectangularly shaped sample of glass, having the dimensions andcomposition of the glass used in Example I, was prepared by polishingits lower side to a finished surface. The sample then was clamped to afixed clamp at one end.

The sample then was heated to a temperature of 550° C. and subjected toa reducing atmosphere consisting of ten percent (10%) hydrogen andninety percent (90%) nitrogen for a period of forty (40) minutes.

The sample temperature was next increased to 600° C. at whichtemperature it began to deform (stretch) under its own weight. Thisstretching was allowed to continue until the sample was stretched to anoverall length of forty (40) inches.

The stretched glass then was placed over a lens shaping fixture, heatedto near the softening temperature of the glass, and was permitted to sagto conform to the desired lens curvation provided by the shapingfixture. The lens blanks then were cut from the fixture and permitted tocool.

Polarization efficiency was measured and found to be forty-three percent(43%) effective.

EXAMPLE III

A rectangularly shaped sample of glass, having the dimensions andcompositions of Example I, was prepared by polishing its lower surfaceto a finished surface. The sample then was clamped to a fixed clamp atone end.

The sample then was heated to its softening temperature range(approximately 600° C.) and, simultaneously, subjected to a reducingatmosphere consisting of ten percent (10%) hydrogen and ninety percent(90%) nitrogen for a period of thirty (30) minutes. At the end of thirty(30) minutes the reducing atmosphere was replaced by a non-reducingnitrogen atmosphere, and the sample heating was continued until it hadstretched to a length of forty (40) inches.

The sample next was placed over a lens shaping fixture, and heated tonear the softening temperature of the glass. The glass was permitted tosag to conform to the desired lens curvation provided by the shapingfixture. The lens blanks were cut from the fixture and permitted tocool.

Polarization efficiency was measured and found to be twenty-eightpercent (28%) effective.

Measurement of polarization efficiency in the foregoing examples wasdone in the following manner. A conventional polarizing filter (a Kaltp.1 φ 52) was placed in front of a light source and rotated; so that thetransmitted light was a minimum. This meant that the transmitted lightwas polarized. The sample produced in each of the above Examples thenwas located in the beam of this polarized light and rotated through360°. The transmission of light passing through the sample was plottedas a function of the angle of rotation to determine the maximumpolarization efficiency according to the following formula:

    P=(T1-T2)/(T1+T2)

where T1 equals the maximum light transmitted through the sample, and T2equals the minimum light transmitted through the sample, as detected bya conventional light meter. No efforts were made in the foregoingExamples to process the glass sample for its optimum degree ofpolarization. The samples were made to determine and illustrate theconcept of the invention.

To ascertain the depth of the penetration of the reducing agent and,therefore, the depth of the elongated polarizing elements in thecompleted stretched sample, the samples were broken to expose across-section through the optical axis. A high powered microscope with acalibrated reticle then allowed the measurement of the depth ofpenetration which, as stated previously, was found to be on the order ofthree to five microns.

The invention has been specifically described in conjunction withpreferred embodiments as set forth in both the general description andin the specific examples. It is to be understood, however, that theExamples given are to be considered illustrative of the invention andnot as limiting. These Examples were not optimized for maximumpolarization efficiency. For example, various changes in the specificdimensions and compositions of the glass will occur to those skilled inthe art. Similarly, various temperatures may be employed withoutdeparting from the concepts of the invention. For example, a relativelywide range of temperatures may be utilized to practice the invention,and nucleation of the reduced metal oxides occur at all of thesetemperatures. The higher temperatures, however, result in fasternucleation than occurs at the lower temperatures. Also, various types ofand compositions of reducing atmospheres may be employed to reach thesame results which are attained in the Examples specifically discussedabove. Such variations will not depart from the true spirit and scope ofthe invention.

I claim:
 1. A method of making polarized ophthalmic glass, comprisingthe steps of:heating a sheet of ophthalmic glass, including a reduciblemetal oxide as a part of its composition, to its softening point in areducing atmosphere for a predetermined period of time sufficient toreduce said metal oxide to metal particles to a predetermined depth lessthan the thickness of said sheet on at least one surface of said sheet;stretching said sheet in one direction to elongate said metal particlesin parallel lines; and cooling the glass to set the elongated metalparticles in the glass.
 2. The method according to claim 1 wherein thereducing atmosphere consists of a hydrogen/nitrogen atmosphere.
 3. Themethod according to claim 2 wherein the temperature to which the glasssheet is heated is three hundred degrees Centigrade (300° C.) to sixhundred degrees Centigrade (600° C.) and the predetermined period oftime is at least ten (10) minutes.
 4. The method according to claim 1further including the steps of removing the reducing atmospherefollowing the heating of said sheet of glass therein for saidpredetermined period of time and holding said sheet of glass in anonreducing atmosphere at its softening temperature for a secondpredetermined period of time prior to stretching said sheet.
 5. Themethod according to claim 4 wherein said second predetermined period oftime is substantially two (2) hours.
 6. The method according to claim 1further including the step of shaping the glass to a predeterminedcurvature following stretching of said sheet and prior to cooling theglass.
 7. The method according to claim 6 wherein said sheet isstretched to elongate it from ten (10) to thirty (30) times its originallength.
 8. The method according to claim 1 wherein said sheet isstretched to elongate it from ten (10) to thirty (30) times its originallength.
 9. A method of making polarized ophthalmic lenses, comprisingthe steps of:heating a sheet of ophthalmic glass, including a reduciblemetal oxide as a part of its composition, to its softening point in areducing atmosphere for a predetermined period of time sufficient toreduce said metal oxide to metal particles to a predetermined depth onat least one surface of said sheet; stretching said sheet in onedirection to elongate said metal particles in parallel lines; placingthe stretched glass sheet over a shaping fixture while said glass sheetis at the softening temperature and permitting said sheet to conform tothe curvature of said shaping fixture; cutting individual lenses fromthe glass after shaping on said shaping fixture; and cooling the glassto set the elongated metal particles in the glass of the individuallenses thus formed.
 10. The method according to claim 9 wherein said atleast one surface comprises the inner surface of each of the lenses thusformed and further including the step of grinding and polishing theouter surfaces of such lenses to particular prescriptions following thecooling of the glass.
 11. The method according to claim 9 wherein thereducing atmosphere comprises a hydrogen/nitrogen reducing atmosphere,and wherein said predetermined period of time is at least ten (10)minutes.
 12. The method according to claim 9 wherein the step ofstretching said sheet takes place after said predetermined period oftime and said sheet is stretched from ten (10) to thirty (30) times itsoriginal dimension in said one direction.
 13. The method according toclaim 9 further including the step of removing the reducing atmosphereafter said predetermined period of time and holding said sheet of glassat said softening temperature for a second predetermined period of timeto nucleate the reduced metal oxides prior to stretching said sheet ofglass.
 14. The method according to claim 9 wherein the glass is heatedto a temperature ranging from three hundred degrees Centigrade (300° C.)to six hundred degrees Centigrade (600° C.) to soften the glass, andwherein said predetermined time is sufficient further to allow thereduced metal oxide to nucleate and wherein said stretching of saidsheet takes place following such nucleation.
 15. The method according toclaim 14 wherein the reducing atmosphere comprises a hydrogen/nitrogenreducing temperature, and wherein said predetermined period of time isat least ten (10) minutes.
 16. The method according to claim 15 whereinthe step of stretching said sheet takes place after said predeterminedperiod of time and said sheet is stretched from ten (10) to thirty (30)times its original dimension in said one direction.
 17. The methodaccording to claim 16 wherein said at least one surface comprises theinner surface of each of the lenses thus formed and further includingthe step of grinding and polishing the outer surfaces of such lenses toparticular prescriptions following the cooling of the glass.